Organic nanocrystals embedded in sol–gel glasses for optical applications

Abstract

We report a simple and generic preparation of stable organic nanocrystals embedded in sol–gel glasses. Nanocrystallization is obtained by instantaneous nucleation followed by controlled growth of the nuclei. In this process, the gel viscosity lowers the growth rate (slow solute diffusion) and inhibits coalescence of the crystals, and pores in the gel act as nanoscale growth reactors. The process is based on control of the nucleation and growth kinetics of the organic phase. For bulk samples, transmission electron microscopy revealed monodisperse particles between 20 and 80 nm in diameter but lower particle diameters are certainly possible due to the nanometer size of pores in dense gel matrix. The generality of this process was demonstrated using various organic molecules selected for their luminescent and optical limiting properties. We have extended this method to the preparation of stable organic nanocrystals embedded in sol–gel thin films using the spin-coating method. These films, which are around 1 mm thick are transparent between 0.3 and 1.5 mm and contain organic crystals characterized by electron and confocal microscopies (100–500 nm in diameter). The targeted properties and applications are electroluminescence, microcavity lasers or nanocrystal spectroscopy. These materials not only combine the optical properties (nonlinear response and fluorescence) of organic molecules with those of inorganic compounds (high stability, wide transparency range) but also combine the advantages of crystals (size effects, photostability) with those of amorphous phases (convenient processing and shaping).